It used to be what this turtle pokemon would yell in a hoarse voice when it squirted water out of it's mouth. Now, it what you yell when you shoot your load into a girl, preferably imitating the hoarse, messed up yet triumphant tone of the turtle pokemon as your dick, squirtle-like, shoots it's fire fighting fluid.

girl: Oh Baby, that feels so good, please don't shoot into me!
guy: Squirtle!
girl: Fuck, you are retarded. And now you are going to be a daddy.

When a male whips out his penis while screaming "squirtle i choose you" then he yells out "use bubble beam" at this stage the male shoots out bubbles made of his ejaculate, which are free to float around and burst in peoples faces.

Person A:Did you see john squirtle at the cinema?
Person B:Yeah, the son of a bitch hit me!
Person A:Suck shit!